1. Field of the Invention
The invention relates to a membrane module for a device for separating substance mixtures in the crossflow process with a plurality of tube membranes which, at least in certain portions, run substantially spirally around a common axis to form a substantially disk shaped module.
2. Description of the Prior Art
Crossflow separating processes operating with membrane modules of this type are used for example for clarifying fruit juices. For this purpose, the unclarified fruit juice is introduced via a feed line into the tube membranes, in the course of which part of the fruit juice having substances of small particle size (down to molecular size), known as the permeate, penetrates the walls of the tube membranes and passes into a permeate tank, while part of the fruit juice having solid substances of large particle size, the retentate, is discharged via a discharge line from the tube membranes and passed on for further processing. In the case of such separation or clarification of the fruit juice, clogging of the walls of the tube membrane acting as filters can be avoided by solid substances that are deposited on the tube walls being detached by the fruit juice flowing through the tube membranes and removed again from the tube membranes.
To obtain a satisfactory filtering performance of the membrane modules used in the crossflow separating processes described, a comparatively large filter area must be provided. Furthermore, it must be ensured that the tube membranes have a sufficiently small inside diameter to obtain a sufficiently small ratio of the interior volume of the tube membranes to the effective filter area that is required to obtain a satisfactory filtering performance. In this case, a satisfactory filtering performance can only he achieved when a plurality of tube membranes are flowed through in parallel. To obtain a compact construction of corresponding separating devices that is required for practical use, U.S. Pat. No. 6,203,707 discloses the use of curved tube membranes instead of straight tube membranes. In the case of a first embodiment of the separating devices described in the cited document, tube membranes which run helically around a common helix axis are used. To avoid damage to the tube membranes, it is proposed in this context to combine the individual tube membranes running around the common helix axis into clusters in which individual tube membranes are twisted in the manner of a rope.
In the case of a further embodiment of the separating devices described in the cited document, the individual tube membranes form membrane modules of the type described at the beginning in which the individual tube membranes run spirally around a common axis to form flat, disk-shaped modules. Membrane modules of this type can be used in particular whenever separating devices of a flat form are required. For this purpose, to obtain a sufficient filtering performance per membrane module, a plurality of tube membranes may be wound spirally into one or more layers parallel to one another.
However, when producing the membrane modules last described, it has been found that the membrane modules obtained using conventional winding devices deviate from the desired shape in many cases, because there are great problems in guiding the individual tube membranes in the winding device. This is attributable to the fact that the diameter of the finished membrane modules is particularly great in comparison with the axial length, so that even small tensile or compressive stresses in the axial direction of the modules cause individual tubes to depart from the winding plane. For this reason, it is generally required when producing the membrane modules described at the beginning for the individual tube membranes to be wound by hand or for particularly complex control techniques to be used. In addition, it is generally also necessary for a radially extending adhesive attachment to be performed between the individual tube membranes at several points within the individual membrane modules during the winding operation. After completion of the known membrane modules of the type described above, it is additionally also necessary for these membrane modules to be handled especially carefully, in order that the individual tube membranes are not displaced from their predetermined position.